1. Field of the Invention
The present invention relates to a scanning probe microscope and to a method of operating the microscope.
2. Description of Related Art
A scanning probe microscope is an instrument capable of measuring the topography of a sample surface at an atomic resolution by scanning the surface using a tungsten probe or cantilever (hereinafter referred to as a probe). FIG. 8 is a diagram showing a feedback circuit incorporated in a scanning probe microscope (SPM). Shown in this figure are a sample 1, a probe 2, a scanner 3 carrying the sample 1 thereon to move the sample in X-, Y-, and Z-directions, a comparator 4 for comparing the output signal from the probe 2 with a reference value, and a Z-drive power supply 5 receiving the output from the comparator 4 and controllably driving the scanner 3 in the Z-direction. For example, a high-voltage (HV) amplifier is used as the Z-drive power supply.
In the circuit designed in this way, a physical amount (such as tunnel current, interatomic force, magnetic force, or electrostatic force) exerted between the probe 2 and the sample 1 is detected. A voltage for control of motion in the Z-direction (Z-motion) is applied to the scanner 3 such that the detected physical amount is kept constant. Thus, motion of the scanner 3 in the Z-direction is controlled. In this way, the sample 1 is moved in the Z-direction (in the heightwise direction) in response to the voltage. The position taken in the Z-direction is controlled.
The sample 1 is moved along the X-Y plane by operating the scanner 3 in the X- and Y-directions while the feedback circuit is in operation. Consequently, a topographic image of the sample surface can be produced. In particular, the voltage at which the scanner 3 is driven is detected as a topography signal corresponding to the topography of the sample surface. In other cases, an electrical current flowing through the probe 2 is detected as a topography signal corresponding to the topography of the sample surface.
A known instrument of this kind (see, for example, Japanese Patent Laid-Open No. H6-265344) has a wide-range measurement device for creating an image of a sample surface and a measuring device for producing a microscopic image of the sample surface by scanning the probe relative to the surface. This technique is characterized in that collision of the probe with convex portions of the sample surface is avoided and that the probe can be moved quickly. In another known technique (see, for example, Japanese Patent Laid-Open No. H10-282127), the probe is scanned along the sample surface while the probe and sample are brought close to each other to such an extent that they affect each other physically. Information about the surface of the sample is obtained based on information about variations in the position of the probe. This instrument has a height-measuring device for measuring the height of the measured surface of the sample to be observed before the probe is brought close to the sample.
In a scanning probe microscope, the probe scans over the sample surface while the distance between the sample surface and the probe is kept constant. However, when the probe scans over tall convex portions on the surface, dust, scratches, edges, or the like, the Z-motion of the sample caused by the scanner may not follow rapid variations in the height of the sample surface. In such cases, there is the danger that dust adheres to the tip of the probe or the probe itself touches the sample surface. If dust adheres to the tip of the probe, it is impossible to perform precise measurements. If the probe touches the sample surface, the probe is damaged, making it impossible to perform measurements.